The present invention relates to AC power control devices, and more particularly, to two terminal incandescent lamp controllers.
The single pole, single throw (SPST) fast acting electrical lighting switch has been the primary device for the control of alternating current to Edison and fluorescent lighting fixtures for over a century. It is simple in operating concept, and has evolved into a highly reliable and inexpensive means of lighting control. Its obvious limitation is the purely binary nature of its control of current. It is either fully on or fully off, except during those initial microseconds prior to closure or following opening. Depending on the inductive or capacitive nature of the applied load, arcing and mechanical damage to the current carrying contacts may occur, thus reducing the long term reliability of the switch. Additionally, as most electrical loads exhibit initially lower impedances when first turned on, the initial in-rush currents to both the switch and the electrical load can be many times higher than the steady state, normal operating currents. This current in-rush phenomena is damaging safety limits of the wiring network and the protective circuit breakers or fuses. Thus, these elements must be greatly over-designed to survive the initial current in-rush conditions.
The past two decades have seen the development of solid state dimmers and mechanical and electronic timer switches intended to directly replace the SPST lighting control switch for commercial and residential control of the Edison gas discharge lamps. Such devices tend to be larger in size than the SPST lighting switch they are intended to replace, and require the homeowner to perform a lengthy procedure to disconnect the power to the effected lighting circuit, remove the previous switch, and install the new device. To many homeowners possessing a healthy fear of electricity, this installation procedure appears dangerous, and thus they would not purchase or install such a device. The primary failure mode of such devices is during the initial turn-on phase, when the in-rush currents of gas discharge lamps can be 16 to 20 times that of the steady state current. As a result, such devices must have very heavy duty current switching elements to withstand this initial electrical stress, which adds significantly to the cost and complexity of the controlling device.
Still other devices exist which act as intermediary fixtures between the incandescent bulb to be controlled and the original receiving fixtures. These devices are usually used to turn on the lamp at the advent of nightfall, or upon sensation of sound or motion. Such devices, while very useful, can only be used in a limited number of fixtures since they add considerable length to the overall bulb to fixture dimensions. Such devices also tend to be unreliable due to the large current in-rushes through the power switching device at initial turn-on.